Proteins with active sites consisting of metal centers bridged by oxo or hydroxo groups comprise a new subclass of metalloproteins. This class includes proteins that perform a variety of functions in biology--dioxygen transport (hemerythrin), the conversion of ribonucleotides to deoxyribonucleotides (ribonucleotide reductase), phosphate ester hydrolysis (purple acid phosphatases), iron storage (ferritin), water oxidation (oxygen evolving complex of Photosystem II), and oxygen activation (methane monooxygenase). We propose to model the structrues and reactivities of such sites using binucleating ligands with phenoxo or alkoxo groups designed to bridge metal centers. Aspects to be modeled include reversible dioxygen binding (hemerythrin), oxygen activation (methane monooxygenase and ribonucleotide reductase), mixed-valent states (hemerythrin, purple acid phosphatans, methane monooxygenase, ferritin), the role of phosphate binding in the purple acid phosphatases and ferritin, ferritin core initiation and nucleation, and properties of the oxygen evolving complex in Photosystem II. The synthetic complexes will be characterized by x-ray crystallography when possible and by a variety of spectroscopic techniques such as NMR, EPR, UV-vis-NIR, Raman, Mossbauer, and EXAFS. Peroxide (and dioxygen) complexes will be studied for their ability to oxygenate or oxidize substrates. Mixed-valent complexes will be investigated with regards to their electron delocalization and spin coupling properties. The synthetic conditions for making iron-oxo and manganese-oxo clusters will be studied.